Liquid crystals are useful for electronic displays because light traveling through a thin film of liquid crystal is affected by the birefringence of the film, which can be controlled by the application of a voltage across the film. Liquid crystal displays (LCDs) are desirable because the transmission or reflection of light from an external source, including ambient light, can be controlled with much less power than is required for luminescent materials used in other displays.
The following terms have the definitions as stated below.                1. Optical axis herein refers to the direction in which propagating light does not see birefringence.        2. Negative C-plate herein refers to the plate in which the optical axis is perpendicular to the plate.        3. In-plane refractive indices is defined by n∥=(nx+ny)/2, where nx, and ny are refractive indices in the direction of x and y, and x-y plane is parallel to the film plane.        4. In-plane birefringence is defined by Δn∥=(nx−ny).        5. In-plane phase retardation is defined by R∥=(nx−ny)d, where d is a thickness of the film in a perpendicular to x-y plane z direction.        6. Out of-plane birefringence is defined by Δn⊥=nz−(nx+ny)/2, where nz refractive index is in z direction.        
7. Out of-plane retardation is defined by R⊥=[nz−(nx+ny)/2]d.
LCDs now are commonly used in such applications as digital watches, calculators, cell phones, portable computers, televisions, and many other types of electronic equipment where the need exists for long life and small room operation with low power consumption. In particular, portable computer and large screen television LCDs benefit from their light-weight, small room occupation, low power consumption, and long life operation. It is expected that LCDs will replace cathode ray tubes (CRT) as monitors and television screens in the near future.
However, there is intrinsic viewing angle dependence in LCDs, which affects the quality of the display performance, such as contrast, coloration, and/or brightness. The primary factor limiting the quality of an LCDs' performance is the propensity of the light to leak through liquid crystal elements or cell, and this leakage's dependence on the direction from which the display is viewed. The best quality LCD picture is observed only within a narrow viewing angle range centered perpendicular to the display screen.
One of common methods to widen LCDs' viewing angles is to apply compensation films. Several LCD modes, including Twisted Nematic (TN), Super Twisted Nematic (STN), Vertical Alignment (VA), and Optically Compensated Bend (OCB), with or without an applied field, show positive C-plate symmetry, which can be compensated for by a compensation film with negative C-plate symmetry.
In a compensation film with negative C-plate symmetry, the out-of-plane refractive index, n⊥ or nz, is less than the in-plane refractive index, n∥=(nx+ny)/2, resulting in a negative out-of-plane birefringence, Δn⊥=nz−(nx+ny)/2<0 and, hence, a negative out-of-plane retardation, R⊥=[nz−(nx+ny)/2]d<0. Negative birefringent films have been prepared by several different methods, such as, but not limited to precision stretching of polymer films, precisely controlled vapor deposition of thin ceramic layers, mixing of a swellable inorganic clay layer in a crosslinked polymer matrix, and solution casting or coating of thin polymer films. For large size negative birefringent films, the solution casting or coating method is preferred due to ease of processing and enhanced performance. A currently used technology involves stretching the film. The drawback to utilizing a stretching of these films involves the resultant stress relaxation which can distort the film, namely at a film/screen's corners. Using a poly(aryletherimide) would eliminate the need for stretching as not only is it nearly impossible to stretch, it is simply not necessary to achieve the results desired.
There are two major ways to apply a negative birefringent film prepared with the casting or coating method onto an LCD component which is an integral part of the LCD device, such as a polarizer. In the first, the negative birefringent film is solution cast on a solvent-passive carrier substrate, adhesive is then applied to the negative birefringent film surface. The combination is laminated on the LCD component and then the carrier substrate is removed (peeled off). In the second case, the negative birefringent film is made by coating the polymer solution directly on an LCD unit component such as a polarizer or a polarizer substrate. This procedure is preferred due to its simplicity and cost saving. However, this procedure requires that the polymer be soluble in select solvents. The solvent must dissolve the polymer which forms the negative birefringent film, but not dissolve or significantly swell the LCD component. The solvent must also be able to be used in large-scale, commercial coating operations. In Japanese patent 3735361, methylisobutyl ketone (MIBK) is shown to be the preferred solvent for solution coating cellulosic substrates since it best meets the above requirements. MIBK also does not dissolve triacetylcellulose (TAC), a commonly used substrate.
Prior art has shown that in order to form a negative birefringent film using solution casting or coating procedures, rigid structural units must be incorporated in the polymer backbone. This is thought to be due to such groups promoting the in-plane orientation of the polymer backbones during the solution casting or coating process. Since the incorporation of rigid groups in a polymer backbone also usually results in a reduction in solubility, special steps must be taken to achieve the desired balance between chain rigidity and solubility. For example, in U.S. Pat. Nos. 5,580,950, and 5,480,964 rigid-rod aromatic polymers, including polyesters, polyamides, and polyimides based on monomers with twisted 2,2′-disubstituted biphenyl structures are utilized. The balance between solubility and backbone rigidity is achieved due to the incorporation of the rigid twisted units in the polymer backbones. The twists in the rigid biphenyl unit hinder chain packing and, thus, enhance solubility.
In U.S. Pat. No. 6,074,709, pendent fluorene groups are incorporated in aromatic polyimide backbones through the polymerization of 9,9-bis(4-aminophenyl)fluorenes in order to attain solubility in useful solvents. However, in order to attain films with negative birefringences >0.01, very rigid dianhydrides, such as 3,3′,4,4′-benzophenonetetracarboxylic acid dianhydride (BTDA), 3,3′,4,4′-tetracarboxylicbiphenyl dianhydride (BPDA) or pyromellitic dianhydride (PMDA), must be used to prepare the polyimide. The use of flexible dianhydrides such as 4,4′-oxydiphthalic anhydride (ODPA) and 2,2-bis(3,4-dicarboxyphenyl)-1,1,1,3,3,3-hexafluoropropane dianhydride (6FDA) result in polyimides that form films with negative birefringences <0.01. These values can only be increased above 0.01 through copolymerization with rigid dianhydrides or rigid diamines such as p-phenylenediamine.
In U.S. Pat. No. 6,853,424 compensator layers are achieved by incorporation of rigid 1,4-dioxophenylene units
in the form of terephthalates. A particularly useful solubilizing monomer is 4,4′-(hexahydro-4,7-methanoindan-5-ylidene)bisphenol, which provides pendent bulky norbornene groups along the polymer backbone that hinder chain packing and enhance solubility, while still maintaining chain rigidity. Solubility can also be enhanced by copolymerization with monomers containing more flexible units such as 1,3-dioxophenylene groups or hexafluoroisopropylidene linkages. Although the use of the flexiblizing comonomer containing hexafluoroisopropylidene linkages (4,4-hexafluoroisopropylidene diphenol) provides suitable solubility, films of poly(terephthalates) prepared with this monomer have negative birefringences of <0.01. Other more rigid comonomers such as 4,4′-(hexahydro-4,7-melhanoindan-5-ylidene) bisphenol must also be used to attain a polyester chain rigid enough to form films with negative birefringences >0.01. Although more rigid than other polyesters, they are not as rigid as polyimides.